The promises of cancer immunotherapy have not been translated into clinical successes because immune-suppressive mechanisms that act in cancer patients can block effective anti-tumor immunity. Therefore, to eradicate cancers by immunotherapy, tumor-induced immunosuppression must be overcome. We have recently demonstrated a novel tumor-intrinsic immunosuppressive mechanism whereby tumor-derived CD73 functions as an ecto-enzyme to produce extracellular adenosine which limits anti-tumor T cell immunity to promote tumor growth via adenosine receptor (AR) signaling. We also found that CD73 expression in malignant cancers was closely associated with poor immune status of tumor infiltrating effector T cells. Our exciting preliminary data using CD73 siRNA-treated tumor cells and CD73-/- mice indicated that ablation of both tumor and host CD73 synergistically inhibited tumor growth in a T cell-dependent manner. A similar anti-tumor effect was observed by pharmacological blockade of CD73 using the selective inhibitor a,-methylene adenosine 5'-diphosphate (APCP). Thus, we hypothesize that both tumor and host CD73 through their enzymatic activity prevent tumor destruction by incoming anti-tumor T cells. We here plan to further clarify the mechanisms of tumor protection by which CD73 expression on either tumor cells or host cells impacts anti-tumor T cell responses. Because endogenous anti-tumor immunity, even if restored, is often insufficient and transient, targeted CD73 cancer therapy may not be optimal unless combined with other forms of immunotherapy, such as adoptive T cell transfer or DC vaccines. Several immunogenic tumors will be tested to assess the efficacy of endogenous and adoptively transferred anti-tumor CD8+ T cell immunity in combination with CD73 ablation. Finally, to establish the translational relevance of targeted CD73 therapy, we will explore the preclinical potential of inhibiting CD73 using APCP or an anti-CD73 monoclonal antibody (mAb) combined with T cell therapy.